In olefin epoxidation, a feed containing an olefin and an oxygen source is contacted with a catalyst under epoxidation conditions. The olefin is reacted with oxygen to form olefin oxide. A product mix results that contains olefin oxide and typically unreacted feed and combustion products, including carbon dioxide. The olefin oxide, thus produced, may be reacted with water to form a 1,2-diol, with an alcohol to form a 1,2-diol ether, or with an amine to form an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers, and alkanolamines may be produced in a multi-step process initially comprising olefin epoxidation and then the conversion of the formed olefin oxide with water, an alcohol, or an amine.
Olefin epoxidation catalysts generally comprise silver, usually with one or more additional elements deposited therewith, on a formed carrier. Carriers are typically formed of a refractory material, such as alpha-alumina. In general, higher purity alpha-alumina has been found to correlate with better performance.
Such catalysts are commonly prepared by a method involving impregnating or coating the formed carrier particles with a solution comprising a silver component and possibly other dopants. The carrier is commonly prepared by forming particles from a dough or paste comprising the carrier material or a precursor thereof and calcining the particles at a high temperature, commonly at a temperature in excess of 900° C.
The performance of the catalysts may be assessed on the basis of selectivity, activity, and stability of operation in the olefin epoxidation. The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. As the catalyst ages, the fraction of olefin converted normally decreases with time. To maintain a desired constant level of olefin oxide production, the temperature of the reaction generally is increased. However, increasing the temperature generally causes the selectivity of the reaction to the desired olefin oxide to decrease. In addition, the equipment used in the reactor typically may tolerate temperatures only up to a certain level. Thus, it may become necessary to terminate the reaction when the reaction temperature reaches a temperature inappropriate for the reactor. Thus, the longer the selectivity may be maintained at a high level and the epoxidation may be performed at an acceptably low reaction temperature while maintaining an acceptable level of olefin oxide production, the longer the catalyst charge may be kept in the reactor and the more product is obtained. Quite modest improvements in the maintenance of selectivity, activity, and stability of operation over long periods may yield huge dividends in terms of process efficiency.
Over the years, much effort has been devoted to improving the performance of olefin epoxidation catalysts. Such efforts have been directed toward improvements to initial activity and selectivity, and to improved stability performance, that is the resistance of the catalyst against aging-related performance decline. In certain instances, improvements have been sought by altering the compositions of the catalysts. In other instances, improvements have been sought by altering the processes for preparing the catalysts, including altering the composition of the carrier and the process for obtaining the carrier.
For example, U.S. Pat. No. 6,858,560-B2 generally discloses a catalyst for the production of ethylene oxide containing silver and a promoting amount of an alkali metal component together with a sulfur component and a boron component. The catalyst is essentially free of rhenium and transition metal components. U.S. Pat. No. 5,663,385 generally discloses an ethylene oxide catalyst comprising silver, a promoting amount of alkali metal, a promoting amount of rhenium, and a promoting amount of a rhenium co-promoter selected from phosphorous, boron, and mixtures thereof, supported on a carrier. GB-1571123 generally discloses a catalyst for the production of alkylene oxides comprising silver, a promoting amount of alkali metal, and a catalyst-life enhancing amount of silicon or aluminum and/or a compound of boron deposited on a porous heat resisting support. These patents do not disclose any benefits obtainable as a result of the order in which boron is added to the carrier/catalyst or the procedures and conditions for such addition.
Not withstanding the improvements already achieved, there is a desire to further improve the performance of olefin epoxidation catalysts.